Using an external device as your Mac’s primary startup device can offer a number of advantages, as we outlined in Using an External Drive as Your Startup Drive: Part 1. This can include having increased storage space, faster performance, the ability to make use of a Mac that has a defective internal drive, or any combination of the above.

In Using an External Drive as your Startup Drive: Part 2, we’ll take a look at additional issues you may encounter while setting up and using an external for startup. We’ll also suggest a few additional choices to consider as candidates for external startup drives.

External Drive Interface Choices

One of the first considerations when choosing an external storage device is how it will interface with your Mac. There are a number of interface types to choose from, but for most of us, the choices will come down to USB 3.1, USB 3.2, and Thunderbolt. You can find more details in the Rocket Yard guide:

Although the ports appear the same, the speed at which they operate can be significantly different.

How to Enable a T2-Equipped Mac to Boot From an External Device

If your Mac includes a T2 security chip, there are a couple of additional steps you’ll need to perform to allow your Mac to start up from an external device. The T2 chip helps protect your Mac against unauthorized access by providing a secure boot environment that ensures the Mac only boots from an authorized (signed) version of the Mac OS. It also prevents booting from external devices that someone may connect to your Mac.

Since our goal here is to boot from an external device, the settings for the T2 security chip will need to be changed. Apple provided the Startup Security Utility so that a user with administrator privileges can alter the default security settings.

The Security Startup Utility is part of the macOS Recovery volume that is hidden on the Mac’s startup drive. For most of us, accessing the recovery volume and it’s Security Startup Utility is just a keyboard shortcut and a restart away.

Access the Recovery Volume

Start by shutting down your Mac.

Once the Mac is off, hold down the Command and R keys (⌘ + R) while starting up your Mac. Continue to hold down the keyboard shortcut until you see the Apple logo. If you’re using a Bluetooth wireless keyboard, you may need to start up your Mac and wait to hold down the keyboard shortcut until after you hear the startup chime. Try both methods if you need to.

The Recovery desktop with macOS Utilities window open.

Once the Apple logo is present, you can release the keyboard shortcut.

After a brief wait, you should see the macOS Utilities desktop appear.

Jump ahead in the instructions to Using the Startup Security Utility

Use Internet Recovery

There’s an alternate method for accessing the recovery volume should the above method fail. When that happens, the usual cause is that the Mac’s startup device has been damaged in such a way as to prevent the hidden recovery volume from being accessible. This usually means the normal startup volume is probably also damaged, which may be why you’re trying to boot from an external to begin with.

If you can’t access the normal recovery volume, you can use a secondary method—Internet Recovery—to gain access to the macOS Utilities desktop and all of its tools.

Internet Recovery results in the same recovery tools being available but doesn’t require a working startup drive. Internet Recovery requires a connection to the Internet via your local network. Once the connection is made, the recovery tools are downloaded to your Mac from an Apple server. If you’re connecting via Wi-Fi, you may need your network password or security key, so be sure and have them handy.

To use Internet Recovery, start up the Mac while holding down the Command, Option, and R keys until you see a spinning globe. Once you see the globe, you can release the keys. It can take a while before the recovery utilities are displayed, so be prepared to wait a bit. When I started my Mac up via Internet Recovery, it took just shy of 5 minutes; your time may be longer or shorter, the point being, go make some coffee or tea while you’re waiting.

Using the Startup Security Utility

No matter which of the two methods you used to start your Mac, you should now be at the macOS Utilities desktop.

The Startup Security Utility allows you to change the default behavior of preventing booting from external devices.

Select Startup Security Utility from the Utilities menu.

You will be asked to authenticate. Click or tap Enter macOS Password, select an administrator account, and enter the password.

If you intend to run a current version of the macOS from the external drive, this is all you will need to do. However, if you’re planning to run an older version of the macOS, you may need to change the choice in the Secure Boot section.

Make your choices and then quit the utility by clicking the red close button in the top left corner of the window.

At this point, you should be able to start up from an external drive that has a bootable macOS volume. If you haven’t yet configured the external with a boot system, jump ahead to: Installing macOS on an External Device.

To select a new startup drive, select Quit macOS Utilities from the macOS Utilities menu.

A sheet will drop down, allowing you to cancel the restart, restart, or Choose Startup Disk.

Select the Choose Startup Disk option.

A list of available startup disks will be displayed. Make your choice and click the Restart button.

The Mac should restart from the selected device.

Installing macOS on an External Device

Depending on the reason that you’re switching to an external drive as your primary startup device, there will be different ways to install the macOS onto the external.

The OWC Mercury Elite Pro Dock includes two internal drive bays along with a number of port expansion capabilities.

Cloning the startup drive: If you’re moving to an external because you need more storage space on the startup drive, or you’re moving to a higher performance device and your current internal startup device is otherwise in good shape, then using a cloning tool to copy of all the information on the current startup drive to the new external may be a good choice. This method will preserve all of your current data, including the system, apps, settings, and documents. Once the cloning process completes, you can boot from the external and pick up right where you left off. Of course, now you will have either a faster startup drive than what your original Mac had, or more storage space; maybe both.

Carbon Copy Cloner: A favorite of the Mac community for creating bootable backups and clones.

SuperDuper: Another user favorite. If you’re working with macOS Catalina, make sure you have the correct version of SuperDuper.

Install a Fresh Copy of the macOS

If you’re upgrading your system with a new bootable external storage system, you may want to consider installing a new, clean copy of the macOS. Once a clean install of the macOS is performed, it will be like the first day you turned on your Mac, only you’re going to have a faster or larger startup drive to use.

macOS installers can be launched from the Recovery or Internet Recovery volumes.

Alternative Method to Install a Clean Copy of the macOS

The Recovery volume has the ability to reinstall the macOS. You can use this method to perform a clean install, provided the version of the macOS you wish to use is the same one you were using before you decided to move to an external boot device.

If you want to install a different version of the macOS, you’ll need to use the methods outlined in the Install a Fresh Copy of the macOS section, above

Use the instructions above in the Access the Recovery Volume section, or the Use Internet Recovery section, depending on the condition of your internal startup disk.

Once the macOS Utilities desktop appears, select the option to Reinstall macOS and then press the Continue button.

After a brief wait, the macOS installer will appear.

The installer will display the name of the macOS version it will install; be sure and confirm it’s the correct version you wish to use.

If it is the correct one, click or tap the Continue button and follow the onscreen instructions.

When you get to the point where you choose the drive to install the operating system on, be sure and choose the external device. After that, you can just follow the instructions provided by the installer.

The OWC Express 4M2 uses Thunderbolt 3 and up to four internal M.2 NVMe SSDs to achieve up to 2800 MB/s performance.

What to Do With the Internal Drive

Using an external as your primary startup device frees up the internal drive your Mac normally uses. If the internal is in good shape, then you have extra storage space you can make use of.

You can put this storage space to work in a number of ways:

Clone of the external startup drive: Having a clone on the internal would allow you to be back up and running quickly should any calamity befall your external drive. If it’s on a portable Mac, the clone will let you leave the external behind if you need to take the Mac on the road. Just remember to clone or sync both startup devices when you get back.

Time Machine backup: If the internal is large enough, it may make a good candidate to be used as your Time Machine backup drive.

Emergency bootable system and troubleshooting tools: If the internal drive is too small for backup use, you may want to use it as an emergency recovery drive. To do this, perform a clean install of a system, then add any troubleshooting tools or apps you need.

Library storage: Depending on the size and possibly the speed of the internal, it may make a good place to keep your various media libraries, ensuring they’re always available by having them stored in the internal drive.

Any combination of the above: If the internal drive is large enough, consider combining the popular uses mentioned above. A Time Machine drive that has a bootable system could be very helpful when you need to restore your data after a device failure.

OWC ClingOn: This may not be a storage device, but it can help ensure your external drive stays connected to its cable. The ClingOn works with Thunderbolt 3 and USB-C cables to keep the cable anchored to the drive, ensuring the cable doesn’t come loose. Say goodbye to data interruptus.

I wish the “Secure Boot” and “External Boot” security options appeared on my iMac when in the Startup Security Utility… but sadly, they don’t. Only the Firmware password protection option shows. I think IT disabled access, although otherwise the machine felt and behaved out of the box. Is there some magical tweak I could do in Terminal or something to get back in the External Boot game?

What about file system impact on external booting? Is an external drive formatted in APFS bootable? Must the external drive be formatted in HFS+ in order for it to be bootable. Since, most internal drives today are not replaceable would that mean that upon failure of the internal that all future operation of what used to be an APFS formatted system would now we operating on HFS+

Great article! I have been thinking of adding an external boot drive for a long time. I have a late 2015 21.5” iMac (iMac,2) with a 1TB fusion drive. I need >=2TB of boot drive storage at the same or better performance than my current boot drive. Any OWC product suggestions?

Please let me clarify. Part 1 doesn’t quite offer a solution. My Mac is an iMac16,2 (sorry for the typo in mt first reply). It does not have TB3 ports nor does it have USB3.1 ports. I do not not want external storage using USB3.0 since it’s performance will be less than what I currently have. Does OWC offer any TB2 solutions?

Yes but the newer macs do not have a startup chime.
So there is no way to startup to Recovery with a wireless keyboard. I usally use a wired keyboard. But if no keyboard is available and bluetooth is not executed – what do I need to do to startup into recovery

I followed the advice above, when I enter the Startup Security Utility I’m asked to authenticate and told that no administrator is found…sounds like it’s tech support time. I miss the old days of easily booting to a drive of your choice using the option key at startup. I’m running a new iMac Pro.

I don’t have one of the new 16-inch MacBook Pros to check for you, but generally speaking I do not think it will work.

The problems is using an older OS with a newer Mac. It is likely that El Capitan will not include the needed drivers and system components to work with the specific hardware contained within the new 16-inch MacBook Pro.

In general, the oldest version of the OS that a Mac model will run is the orginally version that was included when the Mac was released. In the case of the 16-inch MacBook Pro, that would be macOS Catalina.

There have been exceptions in the past, so if anyone can run an older version of the OS on a 16-inch MacBook Pro, let us know.

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